It depends on the junction material, operating temperature, and forward operating current. For high efficienty red LED's, expect a MTBF of >1,000,000 hours! If higher brightness is desired, it could shorten this to 80,000 hours. (which is still almost 8 years, 24/7!)
Blue and White LED's have shorter lifespans. They still last longer than any other (powered) lightsource including HID lighting.
The LED on your mouse will probably last thirty years under normal use. 30 years from now mice will be in history books! :Q
BTW, LED's will have a VERY short lifespan if they get too hot from excessive current. Shuttle mainboards were famous for putting out too much voltage for the power LED. I remember the green power LED's glowing YELLOW before it died.
Cheers!